On the one hand, regions with given hyperdistensions of individual lung regions or pulmonary air cells (alveoli) as well as regions of the lungs in which the pulmonary alveoli have collapsed into themselves, i.e., there are collapses of individual lung regions or pulmonary air cells (alveoli), can be mentioned as lung regions with associated properties.
Apparatuses for electrical impedance tomography are known from the state of the art and are configured and intended for generating an image, a plurality of images or a continuous sequence of images from signals obtained by means of electrical impedance measurements and from data and data streams obtained from such signals. These images or sequences of images show differences in the conductivity of different tissues of the body, such as bones, skin, body fluids and organs, especially the lungs, which are useful for observing the situation of a patient.
Thus, U.S. Pat. No. 6,236,886 describes an electrical impedance tomography apparatus with an array of a plurality of electrodes, a power input at at least two electrodes, a signal acquisition unit at the other electrodes and a method with an algorithm for image reconstruction for determining the distribution of conductivities of a body such as bones, skin and blood vessels in a schematic embodiment with components for signal acquisition (electrodes), signal processing (amplifiers, A/D converters), power input (generator, voltage-to-current converter, current limitation) and components for control functions.
It is explained in U.S. Pat. No. 5,807,251 that it is known in the clinical application of EIT that a set of electrodes is provided as an electrode ring, which set is arranged around the chest of a patient such that its electrodes are spaced at a defined distance from one another in electrical contact with the skin. An electrical current or voltage input signal is applied alternatingly between different electrode pairs or between all of the possible electrode pairs of electrodes arranged adjacent to one another. While the input signal is applied to one of the pairs of electrodes arranged adjacent to one another, the currents or voltages between each pair of the other electrodes, which said pairs are adjacent to one another, are measured, and the data obtained are processed, in order to obtain a visualization of the distribution of the specific electrical resistance over a cross section of the patient, around whom the electrode ring is arranged, and to display this on a display screen.
Unlike other imaging radiological methods (X-ray apparatuses, radiological computed tomographs), electrical impedance tomography (EIT) has the advantage that no radiation burden harmful for the patient occurs. Contrary to sonographic methods, continuous image acquisition can be performed with EIT over a representative cross section of the entire thorax and of the lungs of the patient by means of the electrode belt.
In particular, it is possible by means of EIT to make it graphically possible, in a so-called “tidal image” in the transverse plane of the body, to obtain a visualization of the lungs, showing which regions of the lungs are currently well ventilated and which regions of the lungs are ventilated less well, because the impedances of well ventilated and less well ventilated lung regions differ markedly from one another.
Measured value acquisition of the electrode signals at a scanning rate that makes possible a reconstruction of a sequence of images in order to resolve in time individual breaths, especially the inhalation phases and exhalation phases of a breath, is a basic requirement for analyzing regional distributions of the ventilated air. It is thus possible both to analyze the regional distribution of the ventilated air in the end-inspiratory state and in the end-expiratory state and to examine the characteristics over time during the inhalation and during the exhalation in order to infer lung-mechanical effects, processes and events in the different regions of the lungs.
Lung-mechanical processes and events are, for example, inflow or outflow characteristics of the air caused by flow resistances in the airways and bronchi or bronchioli and alveoli as well as redistributions between different lung regions during the course of the inhalation or exhalation.
Further lung-mechanical effects arise, for example, in case of an excessively high ventilation pressure as well as in case of an excessively low ventilation pressure, so that alveoli are collapsed in some lung regions due, on the one hand, to an excessive distension, (overdistension, hyperdistension, pulmonary emphysema), and, on the other hand, alveoli are collapsed due to an insufficient opening pressure, so that these alveoli are not available for the gas exchange of oxygen and carbon dioxide with the blood circulation.
An example of regional lung-mechanical processes and events is described in the scientific article “Tidal recruitment assessed by electrical impedance tomography and computed tomography in a porcine model of lung injury” [Critical Care Med, 2011, Vol. 40, No. 3]. Different forms of ventilation, such as pressure-controlled ventilation or volume-controlled ventilation, show, together with the settings of the ventilation parameters to be adapted to the patient, such as tidal volume (Vt), respiration rate (RR), inhalation to exhalation ratio (I:E ratio), inhalation pause and exhalation pause, inspiratory pressure (Pinsp), positive end-expiratory pressure (PEEP), a difference in how the breathing gas flows into different regions of the lungs in the course of inhalation over time. Thus, there may be situations in which some regions of the lungs may be hyperdistended, while the opening pressure is too low for some alveoli to open these alveoli and to end the state of collapse in other regions of the lungs at nearly the same time.
The possible effects on the gas exchange are described in the scientific article “Tidal recruitment assessed by electrical impedance tomography and computed tomography in a porcine model of lung injury” [Critical Care Med, Vol. 40, No. 3] that the manner of ventilation thus induces delays in the gas exchange of lung regions, which are due to the so-called “tidal recruitment.” “Tidal recruitment” describes a state of lung regions in which individual collapsed alveoli or a plurality of collapsed alveoli open with a delay only as the pressure increases compared to the other regions of the lungs and close, i.e., collapse again, in turn, prematurely as the ventilation pressure decreases during the exhalation.
How, when and which regions of the lungs are affected by “tidal recruitment” are affected by the settings of the ventilation pressure and of the ventilation pressure curve. Consequently, both a shortened inhalation time and a shortened exhalation time are obtained for these individual collapsed alveoli or for this plurality of collapsed alveoli compared to other regions of the lungs.
Visualization of hyperdistended and collapsed regions of the alveoli (air cells) is advantageous for setting the ventilation parameters and continuously controlling the ventilation parameters. It is described in the scientific article “Bedside estimation of recruitable alveolar collapse and hyperdistension by electrical impedance tomography” [Intensive Care Med, 2009] with respect to the ventilation parameters in terms of the hyperdistension and collapse how regions of the lungs, in which states of excessive distension (hyperdistension) and/or collapse occur, can be identified by means of a simultaneous application of electrical impedance tomography (EIT) and radiological computed tomography (CT). Including values of the ventilation pressure, an image of the best possible or maximum compliance, i.e., the compliance of individual lung regions, is determined for this by means of specific maneuvers with a stepwise reduction of the positive end-expiratory ventilation pressure (PEEP trial) at defined times, at the end of the phase of inhalation and of the phase of exhalation. Compliance is defined in medicine as the quotient of the change in volume to the change in pressure. The unit of measurement is L/kPa, and the unit mL/cm H2O is also frequently used in medicine as well. The impedances or impedance changes determined by means of EIT are used as the equivalent for the change in volume in the electrical impedance tomographic determination of hyperdistension and collapse.
EP 1 292 224 B2 describes a method and a device for the visualization of data which were obtained by means of electrical impedance tomography. Different special modes of analysis, on the basis of which an analysis of the state of the lungs of a patient is intended, are described. Thus, a relative mode is provided, which processes regional changes in a two-dimensional distribution of the ventilation for a past time period, and a phase-shifting mode is used to process the dynamics of ventilation. Furthermore, a perfusion mode is provided, which generates a two-dimensional distribution of the lung perfusion. Further modes described in this EP 1 292 224 B2 are the absolute mode, the time constant mode and a regional spirometry mode. The different modes are used to distinguish different states of the lungs.
It is common to all the modes described in this EP 1 292 224 B2 that no modes and no combination of modes are provided that would make possible a joint visualization of regionally different states of the lungs, such as hyperdistension or collapse.